I also have the 20D but a lot of my shots are not as sharp as I'd like, Lens is a Sigma28-90
The lens currently on the camera is a Canon EF-S 18-55 1:3.5-5.6
I also carry a Canon EF 75-300 1:4-5.6
I used to have a Canon EF-S 18-135mm f/3.5-5.6 IS that was damaged from falling to the floor and bending the focus ring
I could not find a Sigma 28-90 lens with a Canon mount, are you using an adapter?
I would setup a test with the camera on a tripod and a subject 10-12 feet away and manual focus. Measure the exact distance and take some test shots, then move the subject closer/further by 3-6 inches and take another picture at each distance w/o changing the focus ring then evaluate the results.